Method of transferring structured data of constant bit rate traffic in an ATM network

ABSTRACT

A method and an apparatus for transferring structured data of a constant bit rate traffic in an ATM network. The method uses the recurrence of alignment of the structured data field of length N in a 47 octet SA --  PDU payload. At the transmitting end the CSI bit of the header of the first SAR --  PDU is set to 1. It is then set to 0 for the next SAR --  PDUs until the Nth SAR --  PDU is reached. It is then again set to 1. This process is repeated until the last SAR --  PDU has been transmitted.

FIELD OF THE INVENTION

This invention relates to telecommunications networks and moreparticularly to a method of transferring structured data of constant bitrate traffic in an ATM network.

BACKGROUND OF THE INVENTION

In a digital transmission network, data from a large number of users areserially transmitted from one network node to another network node, upto their respective final destinations.

Due to the evolution of networks towards more and more complex mixing ofsub-networks with heterogeneous architectures, it is clear that there isa requirement to support distributed computing applications across highspeed backbones that may be carrying LAN traffic, voice, video andtraffic among channel-attached hosts and workstations.

Fast packet switching is now commonly used to accommodate the bursty,multiprocess communication found in distributed computing environments.

Recently, the concept of cell switching has been introduced. Cellswitching can be thought of as a high performance form of packetswitching. In packet switching networks, the packet size is a fixedmaximum, but individual packets may always be shorter than the maximum.In a cell based network, cells have a fixed length. Cells are usually alot shorter than packets because the use of short fixed length cellssimplifies the hardware needed in each node of the network.

Asynchronous Transfer Mode (ATM) is a protocol for user access to andthe internal operation of a public high speed cell switching system.This protocol is suitable for all kinds of traffic: data, voice, image,video.

In order to make an ATM network practical, it is necessary to adapt theinternal network characteristics to those of the various traffic typesthat will use the network. This is the purpose of the ATM AdaptationLayer (AAL). The function of the AAL is thus to provide generalizedinterworking across the ATM network. The AAL function operates anend-to-end protocol across the ATM network to provide support for endusers of different classes of service corresponding to generic classesof network traffic.

One of these classes (Class One) is intended for constant rate voice andvideo applications. It requires a constant bit rate at source anddestination, a timing relationship between source and destination andpermits to transfer structured information between source anddestination.

Methods that satisfy these requirements are disclosed in RevisedRecommendation I.363 from CCITT, which is included herein by reference.

This Recommendation describes the interactions between the AAL and thenext higher (OSI) layer and the AAL and the ATM layer (sub-layer oflayer 1). The AAL isolates the higher layers from the specificcharacteristics of the ATM layer by mapping the higher layers ProtocolData Units (PDU) into the information field of the ATM cell and viceversa. The AAL entities exchange information with the peer AAL entitiesto support the AAL functions.

To support services above the AAL, some independent functions must beperformed in the AAL. These functions are organized in two logicalsublayers, the Convergence Sublayer (CS) and the Segmentation andReassembly sublayer (SAR).

The SAR prime functions are segmentation of higher layer informationinto a size suitable for the information field of an ATM cell andreassembly of the contents of ATM cell information fields into higherlayer information.

The CS prime function is to provide the AAL service at the AAL ServiceAccess Points (SAP).

The SAR sublayer at the transmitting end accepts a 47 octet block ofdata (SAR₋₋ PDU payload) from the CS sublayer and then adds a one octetSAR₋₋ PDU header to each block to form the SAR₋₋ PDU.

The SAR sublayer at the receiving end receives the 48 octet block ofdata from the ATM layer and separates the SAR₋₋ PDU header. The 47 octetblock of the SAR₋₋ PDU payload is then passed to the CS sublayer. Thebasic AAL1 header is 1 octet long and the payload is 47 octets long.

FIG. 1 represents the SAR₋₋ PDU format of AAL type 1, wherein

SN Sequence Number (4 bits)

SNP Sequence Number Protection (4 bits)

CSI Convergence Sublayer Indicator

SNC Cell Sequence Number (0 to 7)

CRC Cyclic Redundancy Check: corrects single errors

P Parity: detects double errors

The format of the Structured Data Transfer (SDT) method ofRecommendation I.363 is shown in FIGS. 2 and 3. These Figures representthe format of the SAR₋₋ PDU payload. The SAR₋₋ PDU payload used by theCS has two formats, called non-P (FIG. 2) and P (FIG. 3) format. The CSprocedure for SDT uses a pointer to delineate the structure boundaries.The pointer field contains the binary value of the offset, measured inoctets, between the end of the pointer field and the first start of thestructured block in the 93 octets payload consisting of the remaining 46octets of this SAR₋₋ PDU payload and the 47 octets of the next SAR₋₋ PDUpayload.

This method has many disadvantages. The SAR₋₋ PDU header length and theSAR₋₋ PDU payload length are variable. When one cell is lost, thereceiver needs to know if the cell contained 46 or 47 octets of userinformation. The processing is complex. It requires reading of thepointer of P-formats and identification of the start of the structureddata field.

An alternative is described wherein the cells are partially filled. Thepayload is partially filled with user data. The user data length is of Noctets or a multiple of N octets. It guarantees that the first octet ofthe payload is also the first octet of the structured data field. Theformat of the partially filled cell SAR₋₋ PDU payload is shown in FIG.4.

The disadvantages of the method are that it does not apply forstructured data field lengths greater than 47 octets and that the dummydata transported in each cell reduces the efficiency of the ATMconnection.

OBJECTS OF THE INVENTION

It is therefore a principal object of the invention to provide a methodto transfer structured data of constant bit rate traffic in an ATMnetwork, wherein, when a cell is lost during transmission, the receiverdoes not need to know the number of octets of user information containedin the lost cell.

It is another object of the invention to provide a method to transferstructured data of constant bit rate traffic in an ATM network whichdoes not require reading of the pointer of P-formats and identificationof the start of the structured data field.

BRIEF SUMMARY OF THE INVENTION

According to the invention, these objects are accomplished by providinga method and an apparatus for transferring structured data of constantbit rate traffic in a network using the Asynchronous Transfer Mode (ATM)protocol, wherein the ATM Adaptation Layer (AAL) maps the higher layersProtocol Data Units (PDU) into the information field of the ATM cellusing the functions of the Segmentation And Reassembly (SAR) sublayer,said method comprising the following steps:

at the transmitting end,

a) setting the Convergence Sublayer Indicator (CSI) equal to 1, in thefirst SAR₋₋ PDU of the connection, and

b) setting an internal counter to N where N is the length of astructured data field

c) setting CSI=0 in the next SAR₋₋ PDU, and

d) decrementing the internal counter by one,

e) repeating steps c) and d) until the contents of the counter is equalto 1,

f) setting CSI=1 in the Nth SAR₋₋ PDU and resetting the counter to N,

g) repeating steps c) to f) until the last SAR₋₋ PDU of the connectionis transmitted,

at the receiving end,

h) checking that CSI =1 in the first SAR₋₋ PDU received,

i) setting the counter to N,

j) checking that for the next SAR₋₋ PDU having CSI =1 the counter is at1,

k) resetting the counter to N,

l) repeating steps j) and k) until the last SAR₋₋ PDU of the connectionis received.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to a preferred embodimentthereof, which is further shown in the drawings in which:

FIG. 1 shows the SAR₋₋ PDU format of AAL1.

FIG. 2 shows the SAR₋₋ PDU Non-P Format for the Structured Data Transfer(SDT) method.

FIG. 3 shows the SAR₋₋ PDU P-Format for the Structured Data Transfermethod.

FIG. 4 shows the SAR₋₋ PDU format for partially filled cells.

FIG. 5 is a block diagram representing a communications network withinwhich the invention may be practiced.

FIGS. 6A, 6B, and 6C, show the alignment of structured data fields inthe first, the second and the Nth SAR₋₋ PDU payloads, respectively.

FIG. 7 is a flow diagram showing the operations at the transmitting end.

FIG. 8 is a flow diagram showing the operations at the receiving end.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

A packet-switching network usually consists of switching nodes andcommunication links or trunks between these nodes. Each of those nodesis linked to at least one other node through one or more communicationlinks. The switching nodes are data processing systems includingtransmit/receive adapters connected to the communication links.

At each node, incoming data packets are selectively routed to one ormore of the outgoing communication links terminated at another node.Such routing decisions are made in response to information in the headerof the data packet.

In a cell switching network, cells are pieces of data, of fixed length,which are prefixed with headers containing control and routinginformation that identifies the originating and destination users. Eachnode examines each header and decides where to send the cell to move itcloser to its destination.

As described above, Asynchronous Transfer Mode (ATM) is a protocol foruser access to a high speed cell switching network and the ATMAdaptation Layer (AAL) performs functions required by the user, controland management planes and supports the mapping between the ATM layer andthe next higher layer.

FIG. 5 shows a communications network within which the invention can bepracticed. Data Terminal Equipments (DTE) 1 and 2 are linked to ATMnodes 3 and 4, respectively. ATM nodes 3 and 4 are part of ATM network 5and comprise AAL1 5 and AAL1 6, respectively.

The invention will now be described with reference to FIGS. 6A, 6B and6C. As seen before, the AAL function operates an end-to-end protocolacross the ATM network to provide support for the end users of differentservice classes. The services provided by AAL1 to the AAL user are,among others, transfer of service data units with a constant bit rateand delivery of them with the same bit rate and transfer of structuredinformation between source and destination. By structured data it ismeant data transfer with a constant bit rate and a fixed lengthrecurrence (e.g. ISDN primary rate interface contains a 32 octet framerepeated every 125 microseconds).

Let us analyze what is the recurrence of alignment of a structured datafield of length N in a 47 octets SAR-PDU payload.

With

N=length of the structured data field.

R(i)=Remainder of the last structured data field in the SAR₋₋ ₋₋ PDUpayload with a Sequence Number i-1.

R(1)=47-N * Ent (47 / N)

R(i+1)=47-(N-R(i))-N * Ent ((47-(N-R(i))) / N)

When one analyzes the different remainders R(i), when N varies from 2 to50, and i varies from 1 to N, it appears that R(N)=0. This means thatthe alignment of the structured data field in the cell with a SequenceNumber N is the same as the one in the cell with a Sequence Number 1.This also shows that whatever is the value of N, there are always 47structured data fields transported in N cells.

FIGS. 6(A), 6(B) and 6(C) show the alignment of Structured Data Fieldsin the first, the second and the Nth SAR₋₋ PDU payloads, respectively.

FIG. 7 is a flow diagram showing the operations at the transmitting end.The transmitter sets CSI=1 in the first SAR₋₋ PDU of the connection andsets the internal counter to N (61). It then sets CSI=1 and resets thecounter to N in every Nth SAR₋₋ PDU (62) and sets CSI=0 and decrementsthe counter by 1 in every other SAR₋₋ PDU (63). This guarantees thatwhen CSI=1, the first octet of the SAR₋₋ PDU payload is also the firstoctet of the structured data field.

FIG. 8 is a flow diagram showing the operations at the receiving end.The receiver checks that CSI=1 in the first SAR₋₋ PDU of the connectionand sets the internal counter to N. If the first SAR₋₋ PDU has CSI=0, itdiscards the SAR₋₋ PDU and waits for the first SAR₋₋ PDU with CSI=1(71). Then the receiver checks for the next SAR₋₋ PDUs that when CSI=1,it has received N or a multiple of N SAR₋₋ PDUs and resets the counterto N. If not, it inserts dummy SAR₋₋ PDU payloads in order to generate amultiple of N SAR₋₋ PDUs (72).

The advantage of the method is that the SAR₋₋ PDU header length and theSAR₋₋ PDU payload length are constant. When one cell is lost, thereceiver knows that the cell con contained 47 octets of userinformation. The processing is simple. All the information needed iscoded in 1 bit every N cells, where N is the length of the structureddata field.

For the multiplexing of N channels of a T1 or E1 line inside an ATMconnection, an alignment information is provided every 47 * 0.125 ms=5.875 ms, whatever is the value of N.

The method also permits to detect that a multiple of 8 cells are lostwhen the length of the structured data field is not a divider of 8,while current AAL1 does not detect that a multiple of 8 cells is lost.

It also allows accounting of the number of structured data fieldstransported. There are always 47 structured data fields transported in anumber of cells equal to the length of the structured data field.

I claim:
 1. For use in a data communication network in which constantbit rate traffic is to be transported in fixed length cells having afixed length cell header and a fixed length cell data payload, a methodof generating data cells to be transported over a given connectionestablished between a source node and a destination node to allow saiddata cells to be used for transport of N-byte data structures, saidmethod comprising the steps of:a) setting an indicator in a cell headerto a first value and a count to N, filling the cell payload portion withcomplete or partial data structures, forwarding the data cell fortransport, resetting the header indicator to a second value and settingN equal to N-1; b) for each subsequent data cell to be transported overthe same given connection, determining whether N equals 1; c) where N isnot equal to 1, writing any bits of a data structure which was onlypartially loaded into the preceding cell's payload portion into leadingpositions of the current cell's payload portion and filling theremainder of the payload portion with complete or partial datastructures, forwarding the data cell for transport and setting N equalto N-1; and d) repeating steps b) and c) until step b) indicates that Nequals 1 and then repeating steps a) through c) until the last cell tobe transported over the same given connection has been forwarded.
 2. Ina data communication network in which constant bit rate traffic is to betransported in fixed length cells having a fixed length cell header anda fixed length cell data payload, a method of receiving data cells usedin transporting N-byte data structures over a given connectionestablished between a source node and a destination node, each of saidcells having a cell header indicator set to either a first value or asecond value, said method comprising the steps of:a) receiving a datacell; b) determining whether the header indicator in the received cellequals the first value or the second value; c) where the headerindicator in the received cell is found to equal the first value,setting a count equal to N, processing the cell, repeating steps a) andb) and setting N equal N-1; d) where the header indicator in thereceived cell is found to be equal to the second value, processing thecell and determining whether the current count is equal to 1; e) wherethe current count is greater than 1, setting N equal N-1 and repeatingsteps a) through d) until step d) shows that the current count is equalto 1 or until the last data cell has been received; and f) where thecurrent count is equal to 1, repeating steps a)through e) until the lastdata cell has been received.